1. Field of the Invention:
The present invention relates to a novel process for preparing O-alkyl, S,S-dialkylphosphorodithioates, and, more especially, for preparing O-alkyl, S,S-dialkylphosphorodithioates by reacting an alkylphosphate dihalogenide with a thiol.
2. Description of the Prior Art:
O-ethyl, S,S-di-n-propylphosphorodithioate is known to this art as "ethoprophos" and is useful as both an insecticide and nematicide. The current success in the marketplace of compositions containing this compound for the control of insects and nematodes on plants has led to demand for the production of this compound in markedly increasing amounts.
British Patent No. 1,081,270 describes a process for preparing compounds of this type by reacting an alkylphosphate dihalogenide with a thiol (or mercaptan) in the presence of an acid acceptor, in accordance with the following reaction scheme: ##STR1## wherein R.sub.1 and R.sub.2 are alkyl radicals and B is the acid acceptor. This latter material may be a mineral base such as an alkaline hydroxide in solid form, or an organic base such as a tertiary amine. This known process is carried out in a third organic solvent, such as an optionally chlorinated aliphatic or aromatic hydrocarbon, an ether, a ketone or a nitrile. When used for the preparation of ethophrophos, this prior art process provides a yield of 69% in the laboratory, which is far from satisfactory to be economically carried out on an industrial scale.